Map of downtown Boston, Mass., locates the area near the Boston Marathon finish line where two explosions erupted about five hours after the race began; map also shows area landmarks; facts about the incident. MCT 2013. (MCT 2013)

Still image taken from video courtesy of NBC shows an explosion at the Boston Marathon, April 15, 2013. Two explosions struck the marathon as runners crossed the finish line on Monday, witnesses said, injuring an unknown number of people on what is ordinarily a festive day in the city. (REUTERS/NBC/Handout)

An injured man is attended to at the scene of an explosion at the Boston Marathon in Boston, Massachusetts, April 15, 2013. Two explosions struck the marathon as runners crossed the finish line on Monday, witnesses said, injuring an unknown number of people on what is ordinarily a festive day in the city. (REUTERS/Daily Free Press/Kenshin Okubo/Boston University/Handout)

An injured woman is attended to at the scene of an explosion at the Boston Marathon in Boston, Massachusetts, April 15, 2013. Two explosions struck the marathon as runners crossed the finish line on Monday, witnesses said, injuring an unknown number of people on what is ordinarily a festive day in the city. (REUTERS/Daily Free Press/Kenshin Okubo/Boston University/Handout)

Pedestrians walk away from the scene of an explosion near the finish line of the Boston Marathon April 15, 2013. Two people were killed and 23 others injured after two explosions struck the Boston Marathon as runners crossed the finish line on Monday, Boston police said. (REUTERS/Justin McCallum/Tufts University/Handout)

Injured people are attended to at the scene of an explosion at the Boston Marathon, in Boston, Massachusetts, April 15, 2013. Two explosions struck the marathon as runners crossed the finish line on Monday, witnesses said, injuring an unknown number of people on what is ordinarily a festive day in the city. (REUTERS/Daily Free Press/Kenshin Okubo/Boston University/Handout )

Still image taken from video courtesy of NBC shows the scene of an explosion at the Boston Marathon, April 15, 2013. Two explosions struck the marathon as runners crossed the finish line on Monday, witnesses said, injuring an unknown number of people on what is ordinarily a festive day in the city. (REUTERS/NBC/Handout)

An injured man stands at the scene of an explosion at the Boston Marathon in Boston, Massachusetts, April 15, 2013. Two explosions struck the marathon as runners crossed the finish line on Monday, witnesses said, injuring an unknown number of people on what is ordinarily a festive day in the city. (REUTERS/Daily Free Press/Kenshin Okubo/Boston University/Handout)

Law enforcement officials gather outside the John F. Kennedy Presidential Library where a fire has blackened the outside of the building in Boston, Massachusetts April 15, 2013. Boston police said on Monday it was not clear whether two explosions at the Boston Marathon were related to a third explosion that occurred at the John F. Kennedy Presidential Library and Museum, but were investigating any potential connection. (REUTERS/Brian Snyder )

Firefighters and law enforcement officials gather outside the John F. Kennedy Presidential Library where a fire has blackened the outside of the building in Boston, Massachusetts April 15, 2013. Boston police said on Monday it was not clear whether two explosions at the Boston Marathon were related to a third explosion that occurred at the John F. Kennedy Presidential Library and Museum, but were investigating any potential connection. ( REUTERS/Brian Snyder )

Law enforcement officials gather outside the John F. Kennedy Presidential Library where a fire has blackened the outside of the building in Boston, Massachusetts April 15, 2013. Boston police said on Monday it was not clear whether two explosions at the Boston Marathon were related to a third explosion that occurred at the John F. Kennedy Presidential Library and Museum, but were investigating any potential connection. (REUTERS/Brian Snyder )

A woman who apparently sustained an injury to her hand, walks with another who tries to comfort her after explosions reportedly interrupted the running of the 117th Boston Marathon in Boston, Massachusetts April 15, 2013. Two explosions hit the Boston Marathon as runners crossed the finish line on Monday, killing at least two people and injuring 23 on a day when tens of thousands of people pack the streets to watch one of the world's best known marathons. (REUTERS/Dominick Reuter )

People comfort each other after explosions went off at the 117th Boston Marathon in Boston, Massachusetts April 15, 2013. Two explosions hit the Boston Marathon as runners crossed the finish line on Monday, killing at least two people and injuring 23 on a day when tens of thousands of people pack the streets to watch one of the world's best known marathons. ( REUTERS/Jessica Rinaldi )

A Boston Police truck approaches the scene after explosions reportedly interrupted the running of the 117th Boston Marathon in Boston, Massachusetts April 15, 2013. Two explosions hit the Boston Marathon as runners crossed the finish line on Monday, killing at least two people and injuring 23 on a day when tens of thousands of people pack the streets to watch one of the world's best known marathons. ( REUTERS/Dominick Reuter )

A Massachusetts State Police officer stands guard at the scene after explosions reportedly interrupted the running of the 117th Boston Marathon in Boston, Massachusetts April 15, 2013. Two explosions hit the Boston Marathon as runners crossed the finish line on Monday, killing at least two people and injuring 23 on a day when tens of thousands of people pack the streets to watch one of the world's best known marathons. ( REUTERS/Dominick Reuter)

People look for updates about the situation after explosions reportedly interrupted the running of the 117th Boston Marathon in Boston, Massachusetts April 15, 2013. Two explosions hit the Boston Marathon as runners crossed the finish line on Monday, killing at least two people and injuring 23 on a day when tens of thousands of people pack the streets to watch one of the world's best known marathons. (REUTERS/Dominick Reuter )

A woman reacts at the scene after explosions reportedly interrupted the running of the 117th Boston Marathon in Boston, Massachusetts April 15, 2013. Two explosions hit the Boston Marathon as runners crossed the finish line on Monday, killing at least two people and injuring 23 on a day when tens of thousands of people pack the streets to watch one of the world's best known marathons. (REUTERS/Dominick Reuter)

A woman comforts another, who appears to have suffered an injury to her hand, after explosions interrupted the 117th Boston Marathon in Boston, Massachusetts April 15, 2013. Two people were killed and 23 others injured after two explosions struck the Boston Marathon as runners crossed the finish line on Monday, Boston police said. (REUTERS/Dominick Reuter )

A runner is escorted from the scene after explosions went off at the 117th Boston Marathon in Boston, Massachusetts April 15, 2013. Two explosions hit the Boston Marathon as runners crossed the finish line on Monday, killing at least two people and injuring 23 on a day when tens of thousands of people pack the streets to watch one of the world's best known marathons. (REUTERS/Jessica Rinaldi)

A runner in a wheelchair is taken from a triage tent after explosions went off at the 117th Boston Marathon in Boston, Massachusetts April 15, 2013. Two explosions hit the Boston Marathon as runners crossed the finish line on Monday, killing at least two people and injuring 23 on a day when tens of thousands of people pack the streets to watch one of the world's best known marathons. (REUTERS/Jessica Rinaldi )

Agents from the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) work the scene after explosions reportedly interrupted the running of the 117th Boston Marathon in Boston, Massachusetts April 15, 2013. Two explosions hit the Boston Marathon as runners crossed the finish line on Monday, killing at least two people and injuring 23 on a day when tens of thousands of people pack the streets to watch one of the world's best known marathons. (REUTERS/Dominick Reuter )

A woman looks out from a hotel lobby at the scene after explosions reportedly interrupted the running of the 117th Boston Marathon in Boston, Massachusetts April 15, 2013. Two explosions hit the Boston Marathon as runners crossed the finish line on Monday, killing at least two people and injuring 23 on a day when tens of thousands of people pack the streets to watch one of the world's best known marathons. (REUTERS/Dominick Reuter )

Boston Police coordinate the scene after explosions reportedly interrupted the running of the 117th Boston Marathon in Boston, Massachusetts April 15, 2013. Two explosions hit the Boston Marathon as runners crossed the finish line on Monday, killing at least two people and injuring 23 on a day when tens of thousands of people pack the streets to watch one of the world's best known marathons. ( REUTERS/Dominick Reuter )

Public safety officials evacuate the scene after several explosions near the finish line of the 117th Boston Marathon in Boston, Massachusetts April 15, 2013. Two explosions hit the Boston Marathon as runners crossed the finish line on Monday, killing at least two people and injuring 23 on a day when tens of thousands of people pack the streets to watch one of the world's best known marathons. (REUTERS/Neal Hamberg )

People leave the scene after explosions went off at the 117th Boston Marathon in Boston, Massachusetts April 15, 2013. Two explosions hit the Boston Marathon as runners crossed the finish line on Monday, killing at least two people and injuring 23 on a day when tens of thousands of people pack the streets to watch one of the world's best known marathons. ( REUTERS/Jessica Rinaldi )

A child is comforted after explosions went off at the 117th Boston Marathon in Boston, Massachusetts April 15, 2013. Two explosions hit the Boston Marathon as runners crossed the finish line on Monday, killing at least two people and injuring 23 on a day when tens of thousands of people pack the streets to watch one of the world's best known marathons. (REUTERS/Jessica Rinaldi )

Ambulances line the street after explosions reportedly interrupted the running of the 117th Boston Marathon in Boston, Massachusetts April 15, 2013. Two explosions hit the Boston Marathon as runners crossed the finish line on Monday, killing at least two people and injuring 23 on a day when tens of thousands of people pack the streets to watch one of the world's best known marathons. (REUTERS/Dominick Reuter)

A woman walks past an ambulance at the scene after explosions reportedly interrupted the running of the 117th Boston Marathon in Boston, Massachusetts April 15, 2013. Two explosions hit the Boston Marathon as runners crossed the finish line on Monday, killing at least two people and injuring 23 on a day when tens of thousands of people pack the streets to watch one of the world's best known marathons. (REUTERS/Dominick Reuter )

Boston Police and Firemen stand by at the scene after explosions reportedly interrupted the running of the 117th Boston Marathon in Boston, Massachusetts April 15, 2013. Two explosions hit the Boston Marathon as runners crossed the finish line on Monday, killing at least two people and injuring 23 on a day when tens of thousands of people pack the streets to watch one of the world's best known marathons. ( REUTERS/Dominick Reuter )

Public safety officials evacuate the scene after several explosions near the finish line of the 117th Boston Marathon in Boston, Massachusetts April 15, 2013. One or more bombs were responsible for the explosions that struck the Boston Marathon as runners crossed the finish line, two high-level law enforcement sources told Reuters. ( REUTERS/Neal Hamberg )

Spectators and racers cross paths with Boston police officers on Boylston street near the finish line of the Boston Marathon in Boston, Massachusetts April 15, 2013. Two explosions hit the Boston Marathon as runners crossed the finish line on Monday, killing at least two people and injuring 23 on a day when tens of thousands of people pack the streets to watch one of the world's best known marathons. (REUTERS/Scott Eisen )

Boston police officers patrol down Boylston Street near the scene of multiple explosions near the end of the Boston Marathon finish line in Boston, Massachusetts April 15, 2013. Two explosions hit the Boston Marathon as runners crossed the finish line on Monday, killing at least two people and injuring 23 on a day when tens of thousands of people pack the streets to watch one of the world's best known marathons. (REUTERS/Scott Eisen )

A Boston police officer directs people to evacuate the area near the finish line of the Boston Marathon where multiple explosions occurred in Boston, Massachusetts April 15, 2013. Two explosions hit the Boston Marathon as runners crossed the finish line on Monday, killing at least two people and injuring 23 on a day when tens of thousands of people pack the streets to watch one of the world's best known marathons. ( REUTERS/Scott Eisen )

A victim is attended to at the scene of an explosion at the Boston Marathon in Boston, Massachusetts, April 15, 2013. Two explosions struck the marathon as runners crossed the finish line on Monday, witnesses said, injuring an unknown number of people on what is ordinarily a festive day in the city. (REUTERS/Daily Free Press/Kenshin Okubo/Boston University/Handout )

Men in bomb-disposal suits investigate the site of an explosion which went off on Boylston Street during the 117th Boston Marathon in Boston, Massachusetts April 15, 2013. Two explosions hit the Boston Marathon as runners crossed the finish line on Monday, killing at least two people and injuring 23 on a day when tens of thousands of people pack the streets to watch one of the world's best known marathons. ( REUTERS/Jessica Rinaldi )

Emergency medical officials use an ATV to get to the scene of the explosions near the finish line of the Boston Marathon in Boston, Massachusetts April 15, 2013. Two explosions hit the Boston Marathon as runners crossed the finish line on Monday, killing at least two people and injuring 23 on a day when tens of thousands of people pack the streets to watch one of the world's best known marathons. (REUTERS/Scott Eisen )

A runner is escorted from the scene after explosions went off at the 117th Boston Marathon in Boston, Massachusetts April 15, 2013. Two explosions hit the Boston Marathon as runners crossed the finish line on Monday, killing at least two people and injuring 23 on a day when tens of thousands of people pack the streets to watch one of the world's best known marathons. (REUTERS/Jessica Rinaldi )

An injured woman is attended to at the scene of an explosion at the Boston Marathon in Boston, Massachusetts, April 15, 2013. Two explosions struck the marathon as runners crossed the finish line on Monday, witnesses said, injuring an unknown number of people on what is ordinarily a festive day in the city. (REUTERS/Daily Free Press/Kenshin Okubo/Boston University/Handout )

First responders and investigators are near the scene of multiple explosions at the end of the Boston Marathon finish line in Boston, Massachusetts April 15, 2013. Two explosions hit the Boston Marathon as runners crossed the finish line on Monday, killing at least two people and injuring 23 on a day when tens of thousands of people pack the streets to watch one of the world's best known marathons. (REUTERS/Scott Eisen )

New York Police Department officers stand in the Times Square district of New York underneath a news ticker displaying details from fatal explosions in Boston, April 15, 2013. Two explosions hit the Boston Marathon as runners crossed the finish line on Monday, killing at least two people and injuring 23 on a day when tens of thousands of people pack the streets to watch one of the world's best known marathons. (REUTERS/Lucas Jackson )

The scene of multiple explosions near the end of the Boston Marathon finish line in Boston, Massachusetts April 15, 2013. Two explosions hit the Boston Marathon as runners crossed the finish line on Monday, killing at least two people and injuring 23 on a day when tens of thousands of people pack the streets to watch one of the world's best known marathons. ( REUTERS/Scott Eisen )

Bomb squad officials check a possible suspicious device near the scene of multiple explosions close to the end of the Boston Marathon finish line in Boston, Massachusetts April 15, 2013. Two explosions hit the Boston Marathon as runners crossed the finish line on Monday, killing at least two people and injuring 23 on a day when tens of thousands of people pack the streets to watch one of the world's best known marathons. ( REUTERS/Scott Eisen )

Clothing and medical pads are found on Commonwealth Avenue in Boston near the scene of multiple explosions near the end of the Boston Marathon finish line in Boston, Massachusetts April 15, 2013. Two explosions hit the Boston Marathon as runners crossed the finish line on Monday, killing at least two people and injuring 23 on a day when tens of thousands of people pack the streets to watch one of the world's best known marathons. (REUTERS/Scott Eisen)